The present invention relates to apparatus and method for use in oil and gas exploration and production, in particular, but not exclusively, to a burr mill for selectively performing milling and/or burr removal within a well.
When an oil or gas well is drilled it is common to insert a liner or casing into the well in order to support the walls as the depth of the well is increased. In order to access oil or gas containing formation outside the casing, the casing is commonly perforated by means of explosives. As the casing is made of a hardwearing material such as steel, when perforation takes place the steel casing is deliberately damaged to provide access from the wellbore through to the formation and as a result, sections of the casing will be left with exposed metal shards or burrs directed into the wellbore.
Consequently, the insertion of any other tools into the wellbore are susceptible to damage due to collisions with or scraping against the burrs formed during perforation. In particular, delicate screens used for the filtering of fluids downhole can easily be ruptured on contact with the burrs. It would therefore be advantageous to find a method of removing these burrs to avoid damaging tools downhole.
It is already known to attach a mill to a drill string and by rotation of the drill string through the wellbore, burrs may be removed. These tools have the disadvantage that once they have successfully milled off the burrs they become redundant within the well and if left in place they can both cause unwanted wear on the casing and be exerted to unwanted wear on the milling surfaces of the tool as they are subjected to continuous buffering on the inside diameter of the casing.
It is an object of at least one embodiment of the present invention to provide a downhole tool for the removal of burrs or other unwanted debris from inside a wellbore which obviates or mitigates disadvantages in the prior art.
It is an object of at least one embodiment of the present invention to provide a downhole tool in the form of a burr mill which is disengagable so that the milling elements can be removed from the surfaces on which the burrs occur.